1. Hertzsprung-Russell Diagrams

2. What is a star? A cloud of gas, mainly hydrogen and helium The core is so hot and dense that nuclear fusion can occur. The fusion converts light elements into heavier ones

3. Every star is different All the stars in the night sky are different Brightness: – Tells us how luminous the star is, i.e. How much energy is being produced in the core Colour: – Tells us the surface temperature of the star

4. Units of luminosity We measure the luminosity of every day objects in Watts. – How bright is a light bulb? By comparison, the Sun outputs: – 380,000,000,000,000,000,000,000,000 Watts – (380 million million million million Watts!) – This is easier to right as 3.8 x 10 26 Watts To make things easier we measure the brightness of stars relative to the Sun.

5. Units of temperature Temperature is measured in Kelvin The Kelvin temperature scale is the same as the Celsius scale, but starts from -273 o. – This temperature is known as “absolute zero” -273 o C-173 o C0 o C100 o C 0 K100 K273 K373 K 1000 o C 1273 K Kelvin = Celsius + 273

6. Measuring the temperature The temperature of a star is indicated by its colour Blue stars are hot, and red stars are cold Red star 3,000 K Yellow star 5,000 K Blue star 10,000 K

7. Black Body radiation A “black body” is a perfect emitter and absorber of light It emits light at a range of wavelengths which is dependent on its temperature

9. The Hertzsprung Russell Diagram We can compare stars by showing a graph of their temperature and luminosity

10. Luminosity (relative to Sun) 1 100 10,000 0.01 0.0001 Temperature (Kelvin) 25,00010,0007,0005,0003,000 We start by drawing the axes: Luminosity up the vertical axis (measured relative to the Sun) Temperature along the horizontal axis (measured in Kelvin) Where would you mark the Sun on the plot? It has Luminosity of 1 relative to itself Its temperature is 5800 K The stars Vega and Sirius are brighter than the Sun, and also hotter. Where would you put them? Some stars are much cooler and less luminous, such as the closest star to the Sun, Proxima Centauri. Where would you plot these? These stars are called red dwarfs. Sun Sirius Vega Proxima Centauri In fact, most stars can be found somewhere along a line in this graph. This is called the “Main Sequence”. Main Sequence

11. Luminosity (relative to Sun) 1 100 10,000 0.01 0.0001 Temperature (Kelvin) 25,00010,0007,0005,0003,000 Sun Sirius Vega Proxima Centauri Main Sequence Rigel Betelgeuse Deneb Arcturus Aldebaran Sirius B But not all stars lie on the main sequence. Some, such as Arcturus and Aldebaran, are much brighter than the Sun, but cooler. Where would these lie on the diagram? These are orange giant stars. The bright star Betelgeuse is even more luminous than Aldebaran, but has a cooler surface. This makes it a red supergiant. Even brighter than Betelgeuse are stars like Deneb and Rigel, which are much hotter. These are blue supergiants. Some of the hottest stars are actually much fainter than the Sun. Which corner would they be in? These are white dwarfs, such as Sirius B which orbits Sirius.

12. Luminosity (relative to Sun) 1 100 10,000 0.01 0.0001 Temperature (Kelvin) 25,00010,0007,0005,0003,000 Main Sequence Giants Supergiants White Dwarfs Almost all stars we see are in one of these groups, but they don’t stay in the same place. As stars evolve they change in luminosity and temperature. This makes them move around the Hertzprung-Russell diagram. Sun Sirius Vega Proxima Centauri Betelgeuse Arcturus Rigel Deneb Sirius B

13. Luminosity (relative to Sun) 1 100 10,000 0.01 0.0001 Temperature (Kelvin) 25,00010,0007,0005,0003,000 Sun The Sun has been on the Main Sequence for billions of years, and will remain there for billions more. But eventually it will swell into a giant star, becoming more luminous but cooler.

14. Luminosity (relative to Sun) 1 100 10,000 0.01 0.0001 Temperature (Kelvin) 25,00010,0007,0005,0003,000 Sun At this point it is a red giant star. It will get then hotter and slightly brighter, briefly becoming a blue giant.

15. Luminosity (relative to Sun) 1 100 10,000 0.01 0.0001 Temperature (Kelvin) 25,00010,0007,0005,0003,000 Sun Finally nuclear fusion in the core will cease. The Sun will become a white dwarf, far less luminous than its but with a hotter surface temperature.